Solenoids actuators can take many different forms.
A simple single-action solenoid comprises an armature, an electromagnetic coil (often simply referred to as an “electromagnet”), a magnetic core and a spring. Energising the electromagnet causes the armature to move. When the current is switched off, the spring causes the armature to return. Adding a permanent magnet to a single-action solenoid causes the armature to latch. Thus, when the current is switched off, the armature is held in position. To release the armature, the electromagnet is energised with the current flowing in the opposite sense.
A double-action solenoid usually comprises two electromagnets. Dual latching can be achieved by using a permanent magnet, for example, as described in U.S. Pat. No. 4,751,487 A.
In some types of solenoids, the armature is tilted rather than translated linearly. An example of such a solenoid can be found in a balanced armature transducer, such as that described in U.S. Pat. No. 1,365,898 A.
Certain types of solenoid actuators can be used in fuel injectors and engine valves.
For example, US 2007/0095954 A describes a fuel injector having a pintle which is moveable between retracted and extended positions, and a return spring which biases the pintle towards its retracted position. A single-action, non-latching solenoid having an electromagnetic coil and a moveable armature is used to urge the pintle to its extended position. Thus, when the electromagnetic coil is energised, the pintle is urged to its extended position and when the coil ceases to be energised, the pintle returns to its retracted position.
EP 1 837 516 A describes a single-action, non-latching actuator moveable to open and close a fuel valve, a permanent magnet which urges the armature towards a closed position, a spring which urges the armature towards the open position and an electromagnet which produces a magnetic field which interferes with the magnetic field of the permanent magnet which at least reduces the force provided by the permanent magnet on the armature. When the electromagnet is not energised, the permanent magnet exerts a magnetic force to keep the valve in the closed position. When the electromagnet is energised, it creates a magnetic field which reduces the force produced by the permanent magnet. Under the action of the spring, the armature moves to the open position. When the electromagnet is switched off, the force of the permanent magnet closes the valve. Alternatively, the direction of current through the electromagnet may be reversed which helps contribute to the magnetic field closing the valve.
EP 2 194 543 A describes a fuel injector which includes a double-action, non-latching solenoid actuator having an armature, a first electromagnetic coil arranged to move the armature in a first direction, and a spring which helps force the armature in a second direction and retain it in a first (i.e. closed) position. The solenoid actuator also includes a second electromagnetic coil and a permanent magnet associated with the second electromagnetic coil. The permanent magnet produces a magnetic field which acts to move the armature in the second direction and retain the armature in the first position. The second electromagnetic coil generates a magnetic field in the opposite direction to the permanent magnet. Thus, when the second electromagnetic coil is energised, it cancels out the magnetic field of the permanent magnet. At the same time or shortly afterwards, the first electromagnetic coil is energised so as to move the armature in the first direction towards a second position. When the first and second electromagnetic coils cease to be energised, the forces generated by the spring and the permanent magnet act to return the armature back to its first position.
U.S. Pat. No. 5,494,219 A describes a control valve assembly of a fuel injection system having a double-action actuator comprising an armature, first and second coils, and first and second permanent magnets. The armature is held in a first position by the first permanent magnet. The first coil is energised which cancels the magnetic field generated by the first permanent magnet. The second coil is then energised which creates a magnetic field which is in the same direction as the magnetic field generated by the second permanent magnet, thereby pulling the armature towards a second position. The first coil is switched off and, once the armature reaches the second position, the second coil is also switched off. The armature is held in the second position by the second permanent magnet. The process can be repeated, swapping operation of the first and second coils, to return the armature to the first position.
U.S. Pat. No. 5,961,045 A describes a control valve of a fuel injector having a poppet valve member and which includes a single-action solenoid having an armature which includes a permanent magnet and to which the poppet valve member is attached, a coil, and a return spring. Together, the return string and the permanent magnet normally bias the poppet valve member in a first, open position. The permanent magnet is orientated so that when the coil is energised, the permanent magnet pushes away from the coil and flux carrier, pushing the poppet valve into a second, closed position.
EP 1 939 440 A describes a fuel injecting valve having a double-action, dual-latching solenoid which includes a permanent magnet armature arranged between first and second independently-operable coils. The first and second coils are operated to respectively repel and attract the armature and so move a needle valve which is supported by the armature.
Solenoid actuators can also be used to control intake and exhaust valves of a combustion chamber as described, for example, in GB 2 208 041 A (see U.S. Pat. No. 4,779,582 for US counterpart). In this arrangement, a valve closing member is latched into open or closed positions by poles of a permanent magnet against the force of a compressed spring. A coil associated with each position, when activate with a current pulse, cancels the magnetic field of the permanent magnetic poles holding the valve closing member and allows the compressed spring to move the valve closing member quickly through a central neutral position towards the other position.
Another actuator is also described in WO 2005/043266 A and which is used in an ultrafast tool servo. The actuator includes first and second coils, a permanent magnet and an armature supported by flexures.